. >> reporter: day four, president obama comes to boston and speaks at an interfaith service telling boston the country stands with it. >> the world will return to this great american city to run harder than ever and to cheer even louder for the 118th boston marathon. bet on it. >> reporter: later that day, a break in the case. the fbi releases photos and surveillance videos of these two men walking with backpacks. at 10:48 p.m., gunshots are heard on the campus of m.i.t.. reports come in that officer sean collier was killed. soon after, into men car jack a may mercedes leading police on a chase culminating with a shoot-out in watertown where one suspect revealed to be tamerlan tsarnaev was killed. dave 2:40 a.m., a robo call sent to watertown residents. >> advising all watertown residents to remain in their homes. >> reporter: 4:22 a.m., a suspect on the run. >> we believe this to be a terrorist. we believe this to be a man who has come here to kill people. we need to get him in custody. >> reporter: by 8:00 a.m. friday, all of boston and surrounding areas shut down assen aas unprec